Electric-discharge apparatus



` ELECTRIC DISCHARGE APPARATUS,

Filed Oct. 25, 1923 Invehtcgr: Davnd CPrmce,

- His Attoneg.

Patented Aug. 6, 192,9.

STATES 1,723,388 PATENT ortica.

DAVID C. PRINCE, 0F SCHENECTADY, NEW YORK, ASSIGNOR "IO GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

ELECTRIC-DISCHARGE APPARATUS.

Application filed October 23, 1923.

My present invention relates to electricV discharge apparatus, and more particularly to apparatus of the type in which the use ful operating current is carried by means of electrons furnished by a continuously operating ionizing means such for example, as a vapor are.

Many attempts have been made in the past to provide, in electric discharge devices of the ionization type, control means in the form of grids for controlling the flow of current. Such attempts, however, have not resulted in any intensive use of devices of the type in question, largely, I believe, because of the fact that those constructing such devices have failed to understand all of the phenomena upon Which the operation of an ionization device depends and have therefore been unable intelligently to construct a device Which vvould fulfill the operating requirements. l I

The object of my invention is to provide in an electric discharge device a means for controlling the flow of electrons to a collecting electrode or anode in a desired manner, as, for example, entirely preventing cur` rent from flowing to the anode except dur-Y ing predetermined desired intervals.

If an auxiliary electrode of small size be placed in the path of an are discharge and charged positive a few volts a relatively large current of electrons flows to it from the ionized gas so that itbecomes an anode,

sharing the current With the main anode.

VIf the electrode be negatively charged, the

current to it reverses in direction but only a relatively small current flovvsp which increases, in general, slowly, if at all, when the negative voltage is raised to several hundred volts. It has now been demonstrated that this current results almost entirely from. a flow of positive ions to the electrodes.

Let us novv consider the case of a uniformly ionized gas in Which there are sWarms of electrons and ions moving in random directions with velocities corresponding to a fall of potential of say one volt. Assume that each unit of volume contains equal numbers of positive and lnegative Serial No. 670,372.

current density of positive ions which We may denote by I+. The ratio between I and IJr is thus thesame as the ratio of the velocities of the ions, Which is the inverse ratio of the square roots of the masses of the more and more negatively charged, until the number of electrons it receives no longer exceeds the number of posiitive ions. Under the conditionsv We have assumed, with electronsV having a velocity of l volt, the electrode cannot acquire a negative voltage greater than l volt, for if it'did, it could not receive any electrons Iat all, although it would continue to receive positive ions.

Let us now assume that the plane elec trode be charged toa negative potential of 100 volts. Electrons Will therefore be prevented from approaching close to the electrode, Whereas positive ions Will be drawn towards' it.A There Will therefore be a layer of gas near the electrode Where there are positive ions, but no electrons, and in this region there Will therefore be a positive ion Vspace charge. 'The outer edge of this given by Where e is the charge of the electron and m is its mass. If V is expressed in volts, in centimeters, and I in amperes per square centimeter, this equation becomes Equation 1 is applicable also to currents carried wholly by positive ions if we substitute inthe equation the mass of positive ions in place of the mass of the electron. With -mercury ions, therefore, the currents given by the space charge equation are 608 times.

smaller than in the case of corresponding electron currents.

cm.2 By substituting these values of I and V in Equation 2, we find a: equals 0.020 elm. On the other hand, if the voltage of the electrode were 10 volts, m would be 0.0035 cx'n.

- The thickness of the positive ion sheath with 'currentdensities of the order of milliamperes per square centimenter such as those observed in mercury arcs is therefore extremely small; in fact, very small compared with the mean free path of the electrons and ions (2 to 5 cm.). Whether the voltage on the electrode is 10 volts or 100 volts, all theA electrons thatreach the outer edge of the sheath are reflected back into the ionized 4as and all the positive ions that reach this edge of the sheath move towards the electrode and are absorbed by it. The otential of the electrode has noeffect on t e potential distribution in the ionized gas beyond the edge of the sheath, and since the displacenlilent of the edge of the sheath by changing t e negligible compared to the dimensions of the discharge tube, it is clear that the number of positive ions that reach the edge of the sheath is also independent of the potential onthe electrode.

If instead of a planeelectrode we have a small cylindrical electrode, the thickness of the sheath of positive ions will vary"somewhat with the voltage on the electrodes but even with very high negative voltage will not increaseto a value of anywhere near the order' of magnitude of the usual diameter of a discharge tube or bulb. The thickness of the sheath also varies with thev intensity of ionization. If I.r is small, the diameter of the sheath-will be large, the diameter of the sheath will be much less.

-From the above explanation of the phenomena which are present in the re ion sur- 'roundingan'electrode in an ionize gas we are able intelligently to .constructJ device lwith .a grid which is capable ina practical manner of-preventing current from iowing to the-anode. Since the sheath of' positive ions formedl around a grid wire will in an case be of comparatively small diameter, the spacing of the grid wires should be small in order that the sheaths formed around ad-'acent wires may overlap and thus be capa le potential from 10 to 100 volts is enti-rely` where, if I+ is large,

of preventing electrons from lpassing between the grid wires with comparatively small negative voltages on the rid.

Since the thickness of the s ieath also is greater the less intense the ionization, the grid will be most effective if located in a region of the device where the ionization is least intense. Thiswill usually be found in a part which is most remote from the source of ionization althou h the most effective place in this, respect or the location of the grid will of course depend to a large degree upon the structure of the device.

Electrons in moving through the ionized gas make elastic collisions with the atoms of gas and hence move in crooked or random paths. As a result there will be nearly as many electrons moving against the potential' gradient as with it. l Because of this fact it is not suiicient in an ionization device to merely have the grid interposed between the source of ionization and the working anode. The grid must be so arranged that any electrons mustNpass through the grid in order 'to reach the working anode whether moving directly toward the anode or in random directions. l

yIn any case, even with fairly large negative potentials on the grid there will be a small number of electrons which because of their high velocity will pass through the grid. If such electrons in reaching the anode have to travel a distance appreciably greater than their mean free path they may produce by ionization of the gas a sutiicient number of electrons to permit the current to the Working anode to start. The spacing between the grid and working anode therefore should be such that no electrons passing through the grid shall be able to travel a distance appreciably greater than the value of their mean free path before reaching the anode.

These essential requirements may of course be met in numerous ways. I have, found that they seem to be met most effectively by placing the ionizing arc in a main bulb and locating 'the.working anode or anodes in a side arm o1' arms so that the working anode may belocated in a region where the ionization is not too intense. The grid also may to advantage be made of iine wire, closely'spaced, completely surrounding the anod'e and comparatively close thereto.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the` appended claims. My invention itself, however, both as to its organization and method of operation will b estbe understood by reference to the following description taken in connection with the accompanying drawing in which Fig. l shows diagrammatically the application of a .device embodying my invention in a system for converting direct current to alterlll) 'nating; Fig. 2 .is an elevation of the grid construction employed in my device, and Fig. 3 is a partial cross-section of the grid and anode construction.

I have indicated in Fig. 1 an electric discharge device comprising a main bulb 1 having at the bottom a cathode 2 of mercury with anodes?, and 4:, valso of mercury, on either side of the cathode. An arc may be constantly maintained between the cathode 2 and the anodes 3 and 4: by applying be-` tween cathode 2 and these anodes an alternating potential from the source 5, the usual reactance 6 being inserted in the lead to the cathode for maintaining the arc. The bulb 1 is provided with side arms 6 and 7 Working anodes 8 and 9 are sealed into the ends of these side arms. Grids 10 closely surround each anode. The anodes may each be sealed into tubes 11. The grid may be of cylindrical form made by winding a grid wire around supports 12, which are secured to a ring 13 surrounding the tube 11. rThe end of the cylinder adjacent'the anode should also be closed by transverse wires. Leads 14 connected to these rings 13 may be sealed into the ends of the side arms 6 and 7 at the same time that the flare 15 on the tube 11 is joined to the end of the side arm. By this from the negative terminal of a direct current high voltage source is'connected to the cathode 2 and the lead 18 from the positive terminal of the direct current source is connected to the anodes 8 and 9 through an inductance 19 and transformer windings 20 and 21. These windings are shunted by condenser 22 and also by a circuit made up of condensers 23 and a primary 24, which is in inductive relation to a secondary 25, the opposite terminals of which are connected to grids 10. The circuit of the grids is completed from the cathode, grounded lat 26, through ground connection 27 and a resistance 28. Alternating current produced by t-hisl apparatus may be supplied to a distribution circuit from the secondary winding 29,

- which is in inductiverelation to the wind- The manner in which the system operatesy to produce alternating current is not a part of my present invention, being fully described and claimed in my prior application, Serial No. 599,919, filed November 9, 1922.

It is sufficient to say that I have found that current will be prevented from again start-v ing to that anode until the negative potential on the grid has been removed. With the connection shown the grid potentials vary alternately between negative and positive so that when it is desired that current should start to tlow to either one of the anodes the grid surroundingthat anode is made positive. This grid will then of course receive some electrons, but the current flowing in the grid circuit will be limited by the resistance 28 and the main electron current will How to the anode.

As an indication of the capabilities of devices operated in accordance with my invention, vI have constructed a device similar to that illustrated and having a grid inadeofv 10 mil wire spaced 23 mils apart and separated radially from the anodeJ about one l quarter of an inch. With this device I am able by impressing a negative voltage on the grid of 130 volts to prevent current to the anode from starting with a positive potential as high as 45,000 volts on the anode.`

. What I claim as new and desire to secure by Letters Patent of the United States, is

1. The combination of a receptacle, ionizing means in said receptacle, an electrode in said receptacle for collecting electrons, and a grid electrode substantially surrounding said collecting electrode in the region of minimum ionization, said grid being provided with openings of dimensions to prevent electrons from reaching the collecting electrode when it has positive potentials materially greater than the negative potential of said grid electrode.

2. The combination of a receptacle com-4 prising a side arm, ionizing means in said receptacle, an electrode in said side arm for collecting electrons, and a grid electrode surrounding said collecting electrode in the region of minimum ionization, said grid having conductors spaced from one another dis tances to prevent electrons from reaching the collecting electrode when it has positive potentials as high as 10,000 volts and when the grid has negative potentials as low as 100 volts.

3. The combination of a receptacle, ionizing means in said receptacle, an electrode in said receptacle Jfor collecting electron-s, and a grid electrode in the region of minimum ionization substantially surrounding said electrode and provided with openings of such size with respect to the number of positive ions in the neighborhood of the grid that electrons may be prevented from reaching the collecting electrode with positive potentials as high as 10,000 volts on the eollecting electrode and with negative potential of said grid electrode as low as 100 izing means in said receptacle, an electrodo vvolts. in said receptacle for collecting electrons 4. The combination of a receptacle, ionand a grid substantially surrounding said vizing means in said receptacle, an electrode collecting electrode, said grid being spaced Y5 in said receptacle for collecting electrons, from the anode a distance only substan- 15 a grid electrode in said receptacle surroundtially as great as the mean free path of said ing the collecting electrode and comprising electrons. v

1() mil conductors spaced apart by .a distance In Witness whereof, I have hereunto set not substantially greater than 23 mils. my hand this 22nd day of Uctober, 1923. l0 5. The combination of a receptacle, ion- DAVID C. PRINCE. 

